This invention relates to a method of fabricating a fine structure electrode and particularly to a method of fabricating a T-shaped fine structure electrode.
As the integration density and the performance enhancement of semiconductive devices is progressing more and more in recent years, the pattern dimensions are becoming finer and finer, entering into a sub-micron range. At present optical lithography is generally employed to form patterns in the process of fabricating semiconductive devices. However, in fabricating a device like HEMT (high electron mobility transistor) that requires a fine structure electrode of about 0.3 .mu.m as the gate electrode, electron-beam lithography is usually used. The optical lithography has had 0.5 .mu.m as the pattern limit and for a finer structure electrode fabrication, electron-beam lithography and X-ray lithography have been effectively used.
On the other hand, in recent photolithography technology utilizing a projection exposure, research activity has been carried out aggressively on a fine resist pattern forming technology, namely, a so-called phase shifting method. This phase shifting method aims to form a fine pattern of less than 0.5 .mu.m by using existing projection exposure equipment with an improvement applied to a photomask. More specifically stating, this method employs a phase shifting layer that can have the phase of a part of the exposure light after passage of the photomask shifted and can improve the resolving power of the projection exposure method. A brief explanation about the principle of this method is made with the help of FIG. 1(a) through (d) and FIG. 2(a) through (d) in the following:
FIG. 1(a) through (d) show a widely known projection exposure method that uses an ordinary photomask 32 consisting only of a light blocking layer 31 and FIG. 2(a) through (d) shows the projection exposure that uses a phase shifting method using a phase shifting layer 33.
When a photomask 32 with non-phase shifting layer 33 is used as in FIG. 1(a), the phase direction of the light passing through the transparent area of the photomask 32 is not changed as in FIG. 1(b).
Therefore, when a resist pattern having dimensions close to the resolution limit to cause a light diffraction as in FIG. 1(c) is to be formed, the amplitudes of the lights passing through the transparent areas adjacent to each other overlap at the surface of a wafer and the light intensity contrast between the transparent area and the light blocking area observed at the surface of the wafer becomes not as pronounced as described in FIG. 1(d).
On the other hand, when a photomask 34 having a phase shifting layer 33 is used as shown in FIG. 2(a), the phase of the light passing through a transparent area incorporated with the phase shifting layer 33 is turned by 180 degrees against the phase of the light passing the area with no phase shifting layer 33 as shown FIG. 2(b). Thus, if one of the two adjacent areas is covered with a phase shifting layer 33, the amplitudes of the light passing through these transparent areas that are situated almost within a distance of the resolving limit from each other become zero at the surface of the wafer near the intermediate point between the two transparent areas due to the offsetting effect as shown in FIG. 2(d).
Consequently, the resolution power of a projection exposure method is enhanced by employing a phase shifting method whereby the contrast in the light intensity between the areas on the wafer corresponding to the transparent area and the light blocking area of the photomask is enhanced as shown in FIG. 2(d). The phase shifting method as explained in the foregoing is particularly effective in a repetitive formation of a pattern like a line and space pattern.
In contrast to this, as a way to apply the phase shifting method to a pattern of non-repetitive type like the gate pattern of HEMT, there is a method to utilize the end areas of a phase shifting layer of transparent type.
FIG. 3(a) through (d) show the case of a projection exposure wherein the end area of a phase shifting layer of transparent type is utilized. When a photomask 36 with a phase shifting layer 35 is used as in FIG. 3(a), the phase of the light passing through the transparent area with a phase shifting layer 35 formed on it is shifted by 180 degrees at the end of the phase shifting layer 35 as shown in FIG. 3(b). At the surface of a wafer, the light amplitudes overlap due to a diffraction and the light amplitudes near the end of the phase shifting layer are offset to zero as shown in FIG. 3(c). Thus, it is possible to form a fine pattern by having the light intensity reduced to zero at the end of the phase shifting layer 35 as shown in FIG. 3(d).
As described in the foregoing, use of a phase shifting method makes it possible to form a sub-half-micron resist pattern by an ordinary projection exposure method that otherwise has been so far considered impossible.
FIG. 4(a) through (e) show, in cross-sectional views, a process chart of the production method that utilizes a phase shifting method in fabricating a HEMT having a conventional fine structure electrode of T-shaped gates. According to FIG. 4, item 41 is a semiconductor substrate, item 42 is an active area, item 43 is a source drain ohmic electrode, item 44 is a negative photoresist, item 45 is a photomask with a transparent type phase shifting layer 46, item 47 is a fine structure cavity, item 48 is a photoresist, item 49 is a T-shaped resist cavity and item 50 is a gate electrode.
As shown in FIG. 4(a), an active area 42 is formed by mesa etching on a semiconductor substrate 41 having a HEMT structure grown by a MBE method and a source drain ohmic electrode 43 is formed on the foregoing active area 42. Next, as shown in FIG. 4(b), a negative photoresist 44 is applied to all of the surface and this negative photoresist 44 is then developed after it is exposed by a projection exposure to light through a photomask 45 having a transparent type phase shifting layer 46 to form a fine structure cavity 47 of 0.3 .mu.m as shown in FIG. 4(c). Then, as shown in FIG. 4(d), a photoresist 48 is applied to all of the surface and a photoresist cavity of about 0.8 .mu.m is formed by a projection exposure on the foregoing fine structure cavity 47 with an exact position alignment to complete the T-shaped resist cavity 49. After that, as shown in FIG. 4(e), a gate metal such as Ti/Pt/Au for example is applied to all of the surface by deposition and the T-shaped gate electrode 50 is formed by a lift-off method to complete an HEMT.
In the case of a fine structure gate HEMT, a reduction in the gate length causes an increase in the gate resistance with a resultant adverse effect imposed to the device's performance. Thus, it is desirable to make the gate electrode into a T-shape in order to reduce the gate resistance. However, according to conventional phase shifting technology, it is desirable to employ a two-layer resist method in forming a fine structure T-shaped resist cavity for fabricating a fine structure T-shaped gate electrode with a resultant complicated production process and also a problem of pattern defects due to a misalignment of positions at the time of forming an upper-layer cavity.